In a copying device using a binary developing agent, the density of an image to be copied is greatly affected by the amount of toner contained in a developing unit. Therefore, in order to maintain an image density constant in a proper state, toner must be replenished immediately after its consumption in the developing unit. For this purpose, a toner replenishing unit is directly coupled to the developing unit
An operation of the toner replenishing unit is controlled by a toner density control unit consisting of a sensor for detecting short supply of toner in the developing unit and a control section for receiving a signal from the sensor. On the other hand, means for detecting short supply of toner are as follows.
First, a ratio of a toner component in the developing agent is directly measured to detect short supply of toner. In this first method, the toner replenishing unit is activated by the signal from the control section when the ratio of the toner component with respect to a carrier in the developing unit is decreased below a specified value. Second, a latent image of a reference density plate is formed on an image carrier (photosensitive body) to indirectly detect short supply of toner in accordance with a density measurement result of an image (to be referred to as a patch image hereinafter) after development. In this second method, image processing of the patch image having a reference density is performed prior to image processing of a document, and the toner replenishing unit is activated when a measured density of the patch image is lower than a specified density.
However, in the former method, although the toner density can be maintained constant, the image density is not always maintained constant with respect to a change and the like in the latent image formed on the photosensitive body.
On the other hand, in the latter method, although the image density can be advantageously maintained constant, an image of the above reference density plate must be formed prior to formation of each document image. Therefore, this method cannot be applied to a copying device using a document feed unit such as a Recirculating Document Feeder (to be referred to as an RDF hereinafter) which sequentially feeds a large number of documents from a document table to an exposure unit to perform exposure and development, and returns them to the document table again.
That is, when an RDF is used, a document table and an exposure optical system of the copying device are fixed, and documents are continuously fed one by one from the lowermost one to the exposure unit of the document table by a conveyor belt of the RDF, thereby performing the image processing at a high speed, and thereafter the document is returned to its original position and stacked onto the remaining documents, so that the documents are recirculated. On the other hand, in a conventional copying device wherein a document is fixed to perform a copying operation, it is well known that an electrostatic latent image of the above patch image is obtained by adhering the reference density plate of a constant density at a position adjacent to a portion on which the document is placed, exposing the reference density plate similarly to exposure of the document, and forming a latent image of the reference plate on the photosensitive body.
Therefore, in the copying device including the RDF, wherein an exposure optical system is fixed and documents are continuously fed, as it is not possible to form the latent image of the reference density plate on the photosensitive body by scanning the reference density plate, only charging is performed onto the photosensitive body before the document is fed, and unnecessary portions are discharged by a discharging lamp, thereby forming the latent image corresponding to the reference density plate without using the plate. In this method, since a latent image for an image of the reference density plate is formed by a charging effect and turn-on of the discharging lamp during this image formation process, an additional process must be provided to turn off an exposure lamp. Therefore, when a plurality of documents are to be sequentially fed and exposed, a process must be provided to turn off the exposure lamp and to turn it on again every time a document is fed and exposed. As a result, an image processing speed of a document is largely decreased and a merit of using the RDF is lost.
FIG. 1 shows an example of a conventional density control circuit using a reference density plate.
In FIG. 1, reference numeral 1 denotes a toner density sensor. An image of the reference density plate, provided beforehand at the leading end of a platen glass on which a document is placed, is formed on a photosensitive drum, and its developed image is detected by the toner density sensor 1. The toner density sensor 1 is constituted by a photocoupler consisting of an LED 1a as a light-emitting element and a phototransistor 1b as a light-receiving element. Reference numerals VR1 and VR2 denote variable resistors for adjusting a current flowing through the LED 1a; Q1 and Q2, transistors for adjusting a voltage across the variable resistor VR2; Q3, a transistor for amplifying an output from the phototransistor 1b; R1, a base bias resistor of the transistor Q2; R2 and R3, resistors for converting an output current from the transistor Q3 into a voltage; and TH, a thermistor for compensating for temperature changes.
In this circuit, under the condition that a current flowing through the LED 1a is constant (an emitted light amount is constant), if the toner density of a portion to be detected is high, the amount of light reflected at this portion is small, so that the output current from the phototransistor 1b is decreased to reduce the voltage at an output terminal F. On the contrary, if the toner density is low, the voltage is increased.
An A/D converter 2 is connected to the output terminal F so as to convert a detection signal from the sensor 1 into a digital value with a necessary number of bits. The resultant digital value is compared with a reference value in an operation unit 3, and toner is replenished if necessary in accordance with a comparison result. This control is repeated every time a copying operation of one sheet is performed so that the toner density is always maintained constant.
The sensor 1 is also used to detect that a transfer sheet is wound around the photosensitive drum. That is, utilizing the fact that a reflectivity of the transfer sheet is higher than that of the photosensitive drum, a point at which the output from the sensor 1 becomes higher than a normal output when the transfer sheet is wound around the photosensitive drum is detected, thereby stopping the copying operation.
However, as the density of a copy image is increased, the density of a primary image of a toner control patch is also increased. Therefore, the light-emitting element is used at a low sensitivity, so that proper control cannot be performed depending on variations in sensitivity of the light-emitting element. In order to prevent this, a light amount of the light-emitting element may be increased. However, since the light-emitting element is also used as a sensor for detecting winding, an increase in the light amount is naturally limited.